I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

--Mark Hill 21:14, 10 May 2011 (EST) This historic embryology textbook is at only an "embryonic" editing stage with many typographical errors and no figures.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History | Historic Embryology Papers)

The Organs of the Outer Germ-Layer

THE outer germ-layer has for a long time also borne the name dermo-sensory layer. By this its two most important functions are both indicated. For in the first place it forms the epidermis together with its various products, such as hair, nails, scales, horns, and feathers ; and in addition various kinds of glands : the sebaceous, sweat- and milk-glands. Secondly, it is the matrix out of which the nervous system and the most important functional parts of the sensory organs, the optic, auditory, and olfactory cells, are derived.

I begin with the most important function of the outer germ-layer, the development of the nervous system, then proceed to the development of the organs of sense (eye, ear, and organ of smell), and finally discuss the development of the epidermis and its products.

The Development of the Nervous System

The Development of the Central Nervous System

The central nervous system of Vertebrates is one of the organs first established after the separation of the germ into the four primary germ-layers. As has already been stated, it is developed (fig. 41 A) out of a broad band of the outer germ-layer (mp), which stretches from the anterior to the posterior end of the embryonic fundament and lies in the median plane directly above the chorda dorsalis (cli). In this region the cells of the outer germ-layer grow out into long cylindrical or spindle-shaped structures, whereas the elements occurring in the surrounding parts (ep) flatten out and under certain conditions become altogether scale-like. Consequently the outer germ-layer is now divided into two regions into the attenuated primitive epidermis (Hornblatt) (ep) and the thicker median neural or medullary plate (mp).

Both regions are soon sharply separated from each other, since the neural plate bends in a little (fig. 41 B) and its edges rise above the surface of the germ. In this way there arise the two medullary or dorsal folds (rnf), which enclose between them the originally broad and shallow medullary or dorsal furrow. They are simply folds of : the outer germ-layer, formed at the place where the neural plate is continuous with the primitive epidermis. They are therefore composed of an outer and an inner layer, of which the inner belongs to the marginal part of the neural plate, the outer, on the contrary, to the adjacent epidermis.

In all the classes of Vertebrates the medullary plate is transformed into a neural tube at a very early period. This process can be accomplished in three different ways. In most of the classes of Vertebrates, namely Reptiles, Birds, and Mammals, the tube is formed by a typical process of folding. The medullary folds rise still higher above the surface of the germ, then bend together toward the median plane, and grow toward each other until their edges meet, along which they then begin to fuse. The neural tube, thus formed, still continues to remain in connection with the overlying epidermis along the line of fusion, a connection which soon disappears, since the connecting cells become loosened and separated from one another (fig. 41 C}. The closure begins in all Vertebrates at the place which corresponds approximately to the future mid-brain -in the Chick (fig. 87 hb~) on the second and in the Rabbit on the ninth day of development and from there proceeds slowly both backwards and forwards. There is retained for a long time, especially behind, a place where the neural tube is open to the exterior. A connection with the intestinal tube by means of the neurenteric canal also exists at the posterior end, as has been already mentioned (p. 126) in the discussion of the germ-layers. It is only at a later period that this connection is interrupted by the closing of the blastopore.

The second type in the development of the central nervous system is met with in Cyclostomes and Teleosts. In them the neural plate is transformed into a solid cord of cells instead of a tube. Instead of the folds rising up over the surface of the germ, the neural plate grows downward in the form of a wedge. In this way the right and left halves of the plate come to lie immediately in contact with each other, so that one cannot find the slightest trace of a space between them ; only after the cord of cells has been constricted off from the primitive epidermis do the halves separate and allow a small cavity, the central canal, to appear between them. Probably this modification in the Bony Fishes and Cyclostomes is connected with the fact that the egg with its abundant yolk is very closely enveloped by the vitelline membrane, as a result of which the medullary folds cannot rise toward the surface.

The third modification occurs only in Amphioxus lanceolatus. It has already been described briefly in another place (p. 109).

The neural tube retains an undifferentiated condition in Amphioxus lanceolatus only ; in all other Vertebrates, on the contrary, it is differentiated into spinal cord and brain.

The Development of the Spinal Cord

The part of the neural tube which is converted into the spinal cord is oval in cross section (fig. 200). At an early period a separation into a right and left half can be recognised (fig. 232). For the lateral walls are greatly thickened and consist of several layers of long, cylindrical cells, whereas the upper and lower walls are thin and can be distinguished respectively as posterior [dorsal] and anterior commissure (he and vc}, or as roof -plate and floor-plate.

The further development, of which I shall mention only the most important points, takes place in such a manner that the lateral halves become thicker and thicker (fig. 233). The cells continue to increase in number by division, and at the same time to be differentiated into two histological groups (1) into elements which provide the sustentative framework, the epithelium, surrounding the central canal and the spongiosa (spongioblasts of His),, and (2) into elements which are transformed into ganglionic cells and nerve-fibres (neuroblasts of His). The thickening of the lateral walls depends partly upon the multiplication of cells, but mainly upon the fact that nervefibres apply themselves to the cell-mass from the outside. In time these fibres are separated into the anterior, lateral, and posterior columns of the spinal cord (fig. 233 pew, lew, aciv}. At their first appearance the nervefibres are non-medullated (fig. 232 nf), and only subsequently, sometimes earlier, sometimes later, acquire a medullary sheath. In this manner the already considerably thickened halves of the spinal cord become differentiated into the central gray substance containing the ganglionic cells, and into the white substance, which envelops the surface of the former like a mantle.

Since, meanwhile, the roof- and floorplates grow only a little and are not differentiated into
ganglionic cells, they coine to lie deeper and deeper at the bottom of anterior and posterior longitudinal furrows (c and af). Finally, the completely formed spinal cord is composed of large lateral halves, which are separated from each other by deep anterior and posterior longitudinal fissures, being united only deep down by a thin transverse bridge. The latter is derived from the roof- and floor-plates, which have been retarded in their growth, and encloses in its middle the central canal, which has also remained small.

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Fig. 233. Cross section through the spinal cord of an embryo Chick of seven days, after BALFOUR.

At the beginning in Man np to the fourth month of embryonic development the spinal cord occupies the entire length of the body. Therefore, at the time when the axial skeleton is divided up into separate vertebral regions, it reaches from the first cervical down to the last coccygeal vertebra. The end of the spinal cord, however, does not even begin to develop ganglionic cells and nerve-fibres, but remains throughout life as a small epithelial tube. It is united to the larger anterior portion, which has developed nerve-fibres and ganglionic cells, by means of a comcally tapering region, which is spoken of in descriptive anatomy as the conus medullaris.

As long as the spinal cord keeps pace with the vertebral column in its growth, the pairs of nerves arising from it, in leaving the vertebral canal, pass out at right angles directly to the intervertebral foramina. In Man, beginning with the fourth month, this arrangement is changed ; from that time forward the growth of the spinal cord does not equal that of the spinal column, and therefore the cord can no longer occupy the entire length of the vertebral canal. Since it is attached above to the medulla oblongata, and this together with the brain is firmly held in the cranial capsule, it must assume a higher and higher position in the vertebral canal. In the sixth month the conus medullaris is found in the upper end of the sacral canal, at birth in the region of the third lumbar vertebra, and some years later at the lower edge of the first lumbar vertebra, where it terminates even in the adult.

In the ascent (ascensus medulke spinalis) the lower end of the spinal cord, the small epithelial tube which is attached to the coccyx, is drawn out into a long, fine filament, which persists even in the adult as the filwm terminate internum and externum. At first it presents a small cavity, which is lined by ciliated cylindrical cells, and which forms a continuation of the central canal of the spinal cord. Further downward it is continued in the form of a cord of connective tissue as far as the coccyx.

A second consequence of the ascent of the spinal cord is a change in the course of the roots of the peripheral nerve-stems. Since, together with the spinal cord, their points of origin come to lie in the spinal canal relatively nearer and nearer the head, and since the places where they pass through the intervertebral foramina do not change, they are compelled to pass from a transverse to a more and more oblique course. The obliquity, moreover, is greater the farther down the nerve leaves the vertebral canal. In the neck-region their direction is still transverse, in the thoracic region it begins to be more and more oblique, and finally, in the lumbar region, and still more so in the sacral, it is more sharply downward. On this account the nervestems arising from the last part of the spinal cord come to lie for a considerable distance in the vertebral canal before they reach the sacral foramina serving for their exit ; they therefore surround the conus medullaris and filum terminale, forming the structure known as the horse-tail or cauda equina.

Finally the spinal cord undergoes some changes in its form also. Even in the third and fourth months there appear differences of calibre in different regions. The places in the cervical and lumbar regions of the spinal cord at which the peripheral nerves depart to the anterior and posterior extremities, grow vigorously by the abundant formation of ganglionic cells ; they become considerably thicker than the adjoining portions of the cord, on account of which they are distinguished as cervical and lumbar enlargements (intuniescentia cervicalis et lumbalis).

The Development of the Brain

By the study of embryology knowledge of the anatomy of the brain has been greatly promoted. Justly, therefore, in all recent text books of human anatomy, the embryonic condition serves as the starting-point in the description of the intricate structure of the brain, the aim being to derive the complicated ultimate conditions from the more simple embryonic ones, and to explain them by means of the latter.

The initial form of the brain as well as of the spinal cord is a simple tube. At an early period, even before it is everywhere closed, it becomes metameric, on account of its growth being greater in some regions than in others. By means of two constrictions of its lateral walls it is divided into the three primary brain-vesicles (fig. 87 hb l , 7//> 2 , hb 5 ), which remain united with one another by means of wide openings, and are designated as the fore-, mid-, and hind-brain. The posterior of these divisions is the longest, gradually tapering and becoming continuous with the tubular spinal cord.

The first stage is quickly followed by a second, and that by a third, since the primary brain-vesicles soon separate into four, and finally five divisions.

During the second stage (fig. 234) the lateral walls of the primary fore- brain (pvh) begin to grow outward more vigorously and to evaginate to form the two optic vesicles (au). At the same time the lateral walls of the hind-brain, which from the beginning has been the longest portion, acquire a constriction which divides the hindbrain into two vesicles, that of the cerebellum (/,//) and the medulla (??//.), or after-brain.

The five-fold segmentation of the neural tube (fig. 235) soon succeeds the four-fold condition ; by means of it the fore-brain vesicle undergoes fundamental transformations. First, the primary optic vesicles (au) begin to be constricted off from the forebrain vesicle, until they remain attached by only slender, hollow stalks. Since the constriction takes place mainly from above downward, the stalks remain in connection with the base of the fore-brain vesicle. The front wall of the vesicle then begins to protrude anteriorly, and to be marked off by means of a lateral furrow, which runs from above and behind obliquely downward and forward. In this manner the primary vesicle of the fore-brain, like the hind-brain vesicle, is secondarily divided into two portions, which we can now distinguish as the vesicles of the cerebrum and the between-brain
(uh, zh). The optic nerves remain united with the base of the latter. The vesicle of the cerebrum is distinguished by a very rapid growth, and soon begins to surpass all the other parts of the brain in size. But it becomes divided before this into right and left halves. From the connective tissue enveloping the neural tube there grows down in the median plane a process, the future falx cerebri. This growth advances from above and in front against the cerebral vesicle and deeply infolds its upper wall. The halves (fig. 236 hms) that have thus arisen are united at their bases ; they present a more flat median and a convex outer surface, and are called the two vesicles of the hemispheres, since they furnish the foundation for the cerebral hemispheres. The separate regions of the brain-tube produced by constrictions and evaginations subsequently become still more sharply marked off from one another, owing to the alteration of their positions.

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Fig. 234.- Dorsal aspect, by transmitted light, of the head of a Chick incubated 58 hours, after MIHALKOYICS. Magnified 40 diameters.

At the beginning the three brain-vesicles formed by the first constrictions lie in a straight line one behind the other (fig. 87) and above the chorda dorsalis ; the latter extends only as far as to the anterior end of the midbrain vesicle, where it tapers to a point. But from the moment when the optic vesicles begin to be constricted off, the three primary vesicles shift their positions in such a way that the longitudinal axis uniting them undergoes sharp, characteristic folds, which are distinguished as the cephalic, pontal, and nuchal flexures (fig. 235 l-b, nb).

The cause of the formation of the curvatures, which are of fundamental importance in the anatomy of the brain, is to be sought principally in the more vigorous longitudinal growth which distinguishes the cerebral tube, and more especially its dorsal wall, from the surrounding parts. As His has established by means of measurements, the fundament of the brain more than doubles its length, while the spinal cord increases by only about one-sixth of its length.

The cephalic flexure (fig. 235 kb) is developed first. The floor of the fore-brain sinks downward a little around the anterior end of the chorda dorsalis (fig. 237 ch), and forms at first a right angle with the part of the base of the brain lying behind it, but afterwards an acute angle (figs. 235, 238). In consequence of this, the vesicle of the mid - brain (fig. 235 mil} comes to lie highest, and forms a prominence, which causes a great protrusion of the surface of the embryo and is known as the parietal prominence (fig. 158 s).

msp, Longitudinal or interpallial fissure (Man telspalte), at the bottom of which is seen the embryonic lamina terminalis(Schlussplatte) hius, left hemisphere ; zh, between-brain ; mh, mid-brain ; hh, hindbrain and after-brain.

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The nuchal flexure, which makes its appearance at the boundary between medulla oblongata and spinal cord, is less prominent (fig. 235 nb). It produces in the embryos of the higher Vertebrates a curvature which also projects outward, the so-called
nuchal prominence
(fig. 158).

The third curvature, which has been designated by KOLLIKER as the pouted flexure (fig. 239 bb), because it arises in the neighborhood of the future pons
Varolii, is, on the contrary, very marked. It is further distinguished from the two other curvatures described, by the fact that its convexity is not directed toward the back of the embryo, but toward its ventral side.

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Fig. 237. Median section through the head of a Rabbit embryo 6 mm. long, after MIHALKOVICS.

It is formed between the floor of the * [For terminology of the regions of the brain, see footnote, p. 282.]

vesicle of the cerebellum and that of the after-brain, and has the form of a ridge which projects ventrally for a considerable distance, where subsequently the transverse fibres of the pons Varolii are established.

The extent of these curvatures is very different in the various classes of Vertebrates. Thus the cephalic flexure is only slightly emphasised in the lower Vertebrates (Cyclostomes, Fishes, Amphibia) ; it is, on the contrary, much greater in Reptiles, Birds, and Mammals ; but in Man especially, whose brain is the most voluminous, all of the flexures are developed to a very high degree.

The five brain-vesicles furnish the foundation for a natural subdivision of the brain, whose various chief divisions can be referred back to them. As the study of the further development teaches, there are formed from the after-brain vesicle the medulla oblongata, from the vesicle of the cerebellum the vermiform process with the ^hemispheres of the cerebellum and the pons Varolii, from the niidbrain vesicle the crura cerebri and corpora quadrigemina, from the between - brain vesicle the between-brain [thalamencephalon] with the infimdibulum, the pineal gland, and the optic thalami, and finally from the vesicle of the cerebrum the cerebral hemispheres.

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Fig. 239. Brain of a Rabbit embryo 16 mm. long, viewed from the left side. The outer wall of the left cerebrum is removed. After MIHALKOVICS.

In this metamorphosis the cavities of the primitive cerebral tube become the so-called ventricles of the brain : from the cavities of the fourth and fifth vesicles is derived the fourth ventricle or fossa rhomboidalis ; from the cavity of the mid-brain vesicle, the aqueduct of SYLVIUS ; from the between-brain, the third ventricle ; and finally from the cavities of the hemispheres, the two lateral ventricles, which are also designated as the first and second ventricles.

A brief sketch will suffice to show in what manner the most important parts of the brain develop out of the five vesicular fundaments, and that at the same time histological and morphological differentiations are most intimately associated.

Histologically considered the wn^ls of the vesicles originally consist everywhere of closely crowded spindle-shaped cells, just ;is in the spinal cord. These cells undergo in different places unlike modifications. In some places they retain their epithelial character, and furnish (1) the epithelial covering of the choroid plexus in the roof of the between -brain and after-brain, (2) the ependyrna lining the ventricles of the brain, and (3) follicular structures such as the epiphysis (fig. 246). On the greater part of the wall of the five brain-vesicles the cells multiply to an extraordinary extent, and are transformed into more or less extensive layers of ganglionic cells and nerve-fibres. The distribution of the gray and white substances thus formed no longer presents in the brain-vesicles the same uniform condition that it does in the spinal cord. The only uniformity is found in the fact that in every part of the brain there occur gray " nuclei," which, like the anterior and posterior gray columns of the spinal cord, are enveloped with a mantle of white substance, However, there are added to the two parts of the brain that have attained the greatest development layers containing gangliouic cells, which furnish a superficial covering, the gray cortex of the cerebrum and cerebellum. By this means the white substance in certain parts of the brain becomes the core (nucleus medullaris), whereas the gray portion becomes the cortex, a condition differing in an important manner from the structure of the spinal cord.

The morphological differentiation of the brain depends upon the very unequal growth loth of the Jive separate vesicles and of different tracts of their walls. For example, the other four vesicles remain in their development far behind that of the cerebral vesicle, in comparison with which they constitute only a small fraction of the entire mass of the brain (figs. 240, 241). They become overgrown by the cerebral vesicle from above and on the sides, and enveloped as by a mantle, so that they remain uncovered and visible only at the base of the brain. Therefore they, together with a small part of the basal portion of the cerebrum, are grouped together as the stalk of the brain, in contradistinction to the remaining chief part of the cerebrum, which constitutes the cerebral mantle.

The ^mequal growth of the walls of the brain manifests itself in the appearance of thickened and attenuated places, in the development of special nerve-columns (pedunculi cerebri, cerebelli, etc.), and in the formation of more or less extensive layers of ganglionic cells (thalamus opticus, corpus striatum). By these means the principle of the formation of folds, which was fully described in the fourth chapter, is shown to be carried out in a special manner on the hemispheres of the cerebrum and cerebellum inclusive of the vermiform process, that is to say, on the two parts of the brain which are covered with a gray cortex. That the functional capacity of the cerebrum and cerebellum depends upon the extent of the gray cortex and the regularly arranged ganglionic cells in it, is to be concluded from a large number of phenomena. In this way is explained the very extensive increase of surface which is brought about in the cerebrum and cerebellum by means of somewhat different processes of folding. In the cerebrum broad ridges (gyri) arise from the medullary layer of the hemispheres (centrum semiovale), which, running in meandering convolutions, produce the characteristic relief of the surface (fig. 256). In the cerebellum the numerous ridges proceeding from the medullary nucleus are narrow, arranged parallel to one another, and provided with smaller accessory (secondary and tertiary) ridyes, so that the cross section of the cerebellum presents an arborescent figure (arbor vitre).

If, after these preliminary remarks, we take under consideration the metamorphoses of the five vesicles, we may distinguish on each, as MIHALKOVICS has done in his monograph of the development of the brain, four regions : floor, roof, and two lateral parts. We shall begin our description with the fifth vesicle, because in its structure it approaches most closely to the spinal cord.

(1) Metamorphosis of the Fifth Brain-Vesicle.

The ffth brain-vesicle exhibits in different Vertebrates at the beginning of development (in the Chick on the second and third days) faint, regular infoldings of its lateral walls, by means of which it becomes separated into several smaller parts, lying one behind the other. Inasmuch as these afterward disappear without leaving any trace, no great importance was ascribed to them by the earlier investigators (REMAK). Recently, however, several persons have maintained for them a real significance. RABL and BERANECK

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Fig. 241. Brain of a human embryo from the first half of the fifth month, divided in the median plane ; view of the median surface of the right half, after MIHALKOVICS. Natural size.

recognise in them a segmentation of the brain-tube which is related to the exit of certain cranial nerves and is of importance in regard to the question of the metamerism of the entire head-region. The circumstance that the folds are so transitory appears to me to favor the older view.

In the further development of the vesicle of the after-brain a distinction arises between the floor and side walls on the one hand and the roof on the other. The former (figs. 241, 242) are considerably thickened by the addition of nervous substance and become separated on either side of the body (in Man in the third to the sixth months) into columns, which are recognisable from the outside because they are separated by grooves ; these are the extensions with certain modifications of the three familiar columns of the spinal cord. The roof of the vesicle (fig. 235 rf and fig. 243 Dp), on the contrary, produces no nerve-substance, retains its epithelial structure, becomes still thinner, and in the adult consists of a single layer of flat cells. This forms the only covering to the cavity of the dorsoventrally compressed vesicle of the after-brain the fourth ventricle or fossa rhomboidalis. It is firmly applied to the under surface of the pia mater, and with it produces the posterior choroid plexus (tela choroidea inferior). The name choroid plexus has been chosen because the pia mater in this region becomes very vascular and in the form of two rows of branched villi grows into the cavity of the after-brain vesicle, always carrying before it, and thus infolding, the thin epithelial roof.

Laterally the roof-plate or the epithelium of the choroid plexus is continuous with the parts of the brain-vesicle that have been metamorphosed into nervous matter. The transition is effected by means of thin bands of white nervous substance, which, as obex, tsenia sinus rhomboidalis, velum medullare posterius, and pedunculus flocculi, surround the edge of the fossa rhomboidalis. If with the pia mater one strips off from the medulla oblongata the posterior medullary velum, the epithelial covering of the fourth ventricle adhering to the latter will naturally be removed with it. In this way is produced the posterior brain-fissure of the older authors, through which one can penetrate into the system of cavities in the brain and spinal cord.

(2) Metamorphosis of the Fourth Brain-Vesicle.

The wall of the fourth brain-vesicle undergoes a considerable thickening in all its parts, and surrounds its cavity in the form of a ring differentiated into several regions ; the cavity becomes the anterior part of the fossa rhomboidalis (figs. 243, 242, 241). The floor furnishes the 2 )ons (bb), the cross fibres of which become evident in the fourth month. From the lateral walls arise the pedunculi cerebelli ad pontern. But it is the roof that grows to an extraordinary extent and gives to the cerebellum its characteristic stamp. At first it appears as a thick transverse ridye (figs. 242, 243 M), which overhangs the attenuated roof of the medulla. In the third month the
middle portion of the ridge acquires four deep transverse folds by the sinking in of the pia mater (fig. 242), and in this way is distinguished as the vermiform process from the lateral parts, which still appear smooth (kJi). From this time forward the lateral parts outstrip the middle part in growth, bulge out at the sides as two hemispheres, and, acquiring transverse folds, in the fourth month become the voluminous hemispheres of the cerebellum.

Only a little nerve-substance is developed where the roof of the fourth brain-vesicle, which has become thickened to constitute the vermiform process and hemispheres, is continuous with the roof of the third and fifth vesicles (fig. 241). Consequently there arise here thin medullary lamellae, which serve as a transition on the one hand to the posterior choroid plexus, and on the other to the lamina quadrigemina (vJi) the 'posterior and the anterior velum medullare.

Fig. 243. Brain of an embryo Calf 5 cm. long, seen from the side. The lateral wall of the hemisphere is removed. After MIHALKOVICS. Magnified 3 diameters.

(3) Metamorphosis of the Third or Mid-brain Vesicle. (Figs. 235, 243, 242, 241.)
The mid-brain vesicle is the most conservative portion of the embryonic neural tube, the part which is changed least of all ; in Man a small portion only of the- brain is derived from it. Its walls become rather uniformly thickened on all sides of the cavity, which is narrow and becomes the aqueduct of SYLVIUS. The base and lateral walls together supply the crura cerebri and substantia perforata posterior. The roof-plate (fig. 242 vh] becomes the corpora quadrigemina, owing to the appearance, in the third month, of a median furrow, and, in the fifth month, of a transverse one crossing it at right angles.

Whereas at the beginning of the development the mid-brain vesicle (figs. 235, 243 m/i), as a consequence of the curvature of the neural tube, occupies the highest position and produces the parietal prominence of the head (fig. 158 s), it is afterwards covered in from above by the other parts of the brain, which are becoming more voluminous, the cerebellum and cerebrum, and is crowded down to the base of the brain (compare fig. 235 mh with fig. 241 vJi).

Metamorphosis of the Second or Between-brain Vesicle

The between-brain vesicle also remains small, but undergoes a series of interesting changes, since, apart from the optic vesicles, which grow out from its walls, two other appendages, of problematical meaning, are developed from it the pineal gland and the hypophysis.

In the case of the between-brain vesicle, it is only in the lateral walls that a considerable amount of nerve-substance is formed. By this means the walls thicken into the optic thalami with their ganglionic layers. Between them the cavity of the vesicle is retained as a narrow vertical fissure, known as the third ventricle ; it is united with the fossa rhomboidalis by means of the aqueduct of SYLVIUS. The floor remains thin and at an early period becomes evagiiiated downwards ; it thus acquires the form (figs. 235, 241 tr) of a short funnel (infundibulum), with the apex of which is united the hypophysis, soon to be fully described.

The roof presents in its metamorphosis a striking similarity to the corresponding part of the after-brain vesicle (fig. 241). It persists as a simple, thin epithelial layer, unites with the very vascular pia mater, which sends out in this case also villotis outgrowths with capillary loops which pass into the third ventricle, and together with it constitutes the anterior choroid plexus (tela choroidea anterior or superior). When in withdrawing the pia mater the choroid plexus is also removed, the third ventricle is opened ; thus is produced the anterior great fissure of the brain through which, as through the structure of the same name in the medulla oblongata, one can penetrate into the cavities of the brain.

The agreement with the medulla oblongata is expressed in still another point. As in the case of the latter the edges of the roofplate develop into thin medullary bands, by means of which the attachment to the sides of the fossa rhomboidalis is accomplished, so here also the epithelium of the choroid plexus attaches itself to the surface of the optic thalanius by means of thin bands consisting of medullated nerve-fibres (taenise thalaini optici).

Finally, out of the hindermost portion of the roof of the betweenbrain vesicle a peculiar organ, the pineal <jland (fig. 241 s), takes its origin at a very early period, in Man in the course of the second month. Since in recent years numerous interesting works have appeared concerning it, and since many striking discoveries have been brought to light both in the case of the Selachians and more especially in that of the Reptiles, I will describe it at somewhat greater length.

The Development of the Pineal Gland (Epiphysis cerebri).

First it is to be mentioned that, with the exception of Amphioxus lanceolatus, the pineal gland (glandula piiiealis s. conariuin) is not wanting in any Vertebrate. It is in all cases formed in exactly the same way. On the roof of the between-brain, where it is continuous with the roof of the mid-brain or the lamina quadrigemina, there arises an evagination (figs. 238 and 241 z) which has the shape of the finger of a glove, the processus pinealis [epip/iysis cerebri], the apex of which is at first directed forward, but subsequently backward. In its further metamorphosis there appear, as far as our knowledge at present extends, differences of considerable importance.

According to the investigations of EHLERS, the pineal process attains in adult /Selachians an unusual length ; its closed end swells into a vesicle, which penetrates the cranial capsule and extends out to the dermal surface. In many Selachians, such as Acanthias and Raja, the vesicular end is enclosed in a canal of the cranial capsule itself ; in others it lies outside between the cranial capsule and the corium. The [proximal] end of the vesicle is united to the betweenbrain by means of a long slender canal.

Manifold conditions are met with in Reptiles, as the recent investigations of SPENCER have taught. These conditions permit in part a direct comparison with the Selachians, but in part they are widely altered. Here, too, the pineal gland is a structure of considerable length, the peripheral end of which lies far away from the betweenbrain under the epidermis; it passes out through an opening in the roof of the skull which is situated in the parietal bone and is known as the foramen parietale. The position of the latter can easily be determined on the head of the living animal) because at this place the dermal scutes acquire a special condition and form, and, above all, are transparent.

In regard to the particular form of the organ, there are essentially three types to be distinguished.

In many Reptiles, e.g., in Platydactylus, the pineal gland has the same structure as in Sharks : a small peripheral vesicle, which is
schb p st bl x p

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Fig. 244. -Diagrammatic longitudinal section through the brain of Chameleo vulgaris with the pineal organ, which is separated into three portions, a vesicular, a cord-like, and a tube-like portion, after BALDWIN SPENCER.

enclosed in the parietal foramen, is lined with ciliated cylindrical cells, and is connected with the roof of the between-brain by means of a long, hollow stalk.

In other Reptiles, as in the Chameleon, the organ is differentiated into three portions (fig. 244) : first into a small closed vesicle (bl), which lies under a transparent scale (x) in the foramen parietale and is lined with ciliated epithelium ; secondly into a solid cord, which consists of fibres and spindle-shaped cells, and bears a certain resemblance to the embryonic optic nerve j and thirdly into a hollow, funnel-shaped projection (J) of the roof of the between brain, which still exhibits here and there sac-like enlargements.

In a third division of the Hep tiles, in Hatteria, Monitor, the Blind-w o r m s, and Lizards,
A
the vesicular distal portion i of the pineal k gland undergoes a striking r metamo r p h osis, by means of which it acquires a certain resemblance to the eye of many Invertebra t e s (fig. 245). The portion of its wall which lies next to the surface of the body has been transformed into a lens-like structure (1} ; the part of the wall lying opposite the latter and continuous
with the fibrous cord (Si) has, on the contrary, been converted into a retina-like structure (r~). The formation of the lens (I) is clue to the fact that the epithelial cells of the anterior wall of the vesicle have become elongated into cylindrical cells and uninucleate fibres, and have thereby produced an elevation, the convex surface of which projects into the cavity of the vesicle. In the posterior portion the epithelial cells are separated into different layers, the innermost of which is distinguished by the abundance of its pigment. Between the pigmented cells there are imbedded others, which can be compared to the rods of the visual cells in the paired eyes of Vertebrates, and which appear to be in connection below with nerve-fibres.

Those investigators who, like RABL-RUCKHARD, AHLBORN, SPENCER, and others, have studied the pineal gland, are of opinion that the pineal body must be considered as an unpaired parietal eye, which in many classes, for example in Reptiles, appears to be tolerably well preserved, but in most Vertebrates is in process of degeneration.

That we have to do in Reptiles with an organ which reacts under the influence of light, does not appear improbable, when one takes into consideration that, owing to the transparency of the dermal scutes at the place in the skull where the parietal foramen is located, rays of light are here able to penetrate through the integument. The presence of a lens-like body and pigment is also favorable to this view. But whether the organ serves for sight, or only for the transmission of sensations of warmth, whether, consequently, it is more an organ for the perception of warmth than an eye, must for the present remain undecided. It is still more an open question whether this organ of warmth is a structure which has been developed as a special modification of the epiphysis of Reptiles alone, as the auditory sac, for example, has been developed in the tail of the Crustacean Mysis, or whether it represents a structure originally common to all Vertebrates. In the latter case processes of degeneration must be assumed to be widespread, for up to the present time nothing like the condition in Reptiles has been found in other Vertebrates.

In Birds and Mammals the pineal process undergoes metamorphoses which give rise to an organ of a glandular, follicular structure.

In Birds (fig. 246) it never attains such great length as in Selachians and Reptiles. At a certain stage it sends out from its surface into the surrounding vascular connective tissue cellular outgrowths, which increase in number by means of budding and finally break up into numerous small follicles (fig. 246 /). These consist of several layers of cells, the outermost being small, spherical elements, the innermost cylindrical ciliated cells. The proximal portion of the pineal process does not become involved in the follicular metamorphosis and persists as a funnel-shaped outfolding of the roof of the between-brain ; the individual follicular vesicles constricted off from the parental tissue are ' united with its upper end by means of connective tissue.

In Mammals the development takes place in a manner similar to that of the Chick. In the Rabbit there also arise follicles, each of
which at first encloses a small cavity, but later becomes solid. They are then entirely filled with spherical cells, which possess a certain resemblance to lymphcorpuscles. The opinion has therefore been expressed by many (HENLE) that the pineal body is a lymphoid organ, an opinion, however, which is refuted by the study of the development, for genetically the follicles are exclusively epithelial structures.

In the adult there are formed
within the individual follicles concretions, the brain-sand (acervulus cerebri).

In Man the pineal body, which begins to appear in the sixth week (His), exhibits a peculiarity as regards its position. Whereas the free end of the epiphysis is at first directed forward, and in other Vertebrates is also retained in this position, it acquires in Man an opposite direction, inasmuch as it bends backward on to the surface of the lamina quadrigemina. Probably this is connected with the fact that the gland is crowded back by the excessive development of the corpus callosum.

As the signification of the pineal gland is still doubtful, so is that of the pituitary body or hypophysis cerebri, which, as has been previously mentioned, is united with the floor of the bet ween -brain at the apex of the infundibular process.

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Fig. 246. Section through the pineal gland of a Turkey, after MIHALKOVICS. Magnified ISO diameters.

/, Follicle of the pineal gland with its cavities ; b, connective tissue with blood-vessels.

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The Development of the Hypophysis (Pituitary Body)

The hypopliysis is an organ which has a double origin. This is expressed in its entire structure, since it is composed of a larger, anterior and a smaller, posterior lobe, which in their histological characters are fundamentally different from each other.

In order to observe the beginning of its formation, it is necessary to go back to a very early stage (fig. 237), in which the oral sinus has just arisen and is still separated from the cavity of the head-gut by means of the pharyngeal membrane (rh). At this time the cephalic flexure of the brain-vesicles has already appeared, and the anterior end of the chorda dorsalis (ch) terminates immediately behind the attachment of the pharyngeal membrane. In front of this is located the important place where the hypophysis is developed, as was first established by GOETTE and MIHALKOVICS. The hypophysis is therefore a product of the outer germ-layer and not a growth from the cavity of the head- gut, as had always been maintained previous to this time.

The first steps introductory to the formation of the hypophysis take place soon after the rupture of the pharyngeal membrane (figs. 238, 247), some unimportant remnants of which are retained at the base of the skull as the so-called primitive velum palatinum. Anterior to these there is now developed (in the Chick on the fourth day of incubation, in Man during the fourth week, His) a small evagination, the pouch of RATHKE or the pocliet of the hypophysis (%), which grows toward the base of the b e t w e e n-b rain (tr). Then it becomes deeper, begins to be constricted uoff from its parent tissue, and to be metamorphosed into a small sac, whose wall is composed of several layers of cylindrical cells (fig. 248). t ./ 6
The sac of the hypophysis (hy) remains for a long time in connection with the oral cavity by means of a narrow duct (hyy). In later stages, however, the connection in the higher Vertebrates interrupted, because the embryonic connective tissue, which supplies the foundation for the development of the skeleton of the head, becomes thickened and crowds the sac farther away from the oral cavity (figs. 248, 249). When, later on, the process of chondrification takes place in the connective tissue, by means of which the cartilaginous base of the skull (sc/w) is established, the sac of the hypophysis (%) comes to lie above the latter at the under surface of the between-brain (tr). At this time also the duct of the hypophysis (hyg), which meanwhile has lost its lumen, begins to shrivel and degenerate (fig. 249). In many Vertebrates, however, as in the

tr, Floor of the between -brain with infundibulum ; hy, hypophysis ; hy', part of the hypophysis in which the formation of the glandular tubules begins; hyg, duct of the hypophysis; schb, base of the skull ; ch, chorda ; si, dorsum sellae.

tr, Floor of the between-brain with infundibulum ; hy, original pouch like part of the hypophysis ; Inj', the glandular tubules which have budded out from the sac of the hypophysis ; si, dorsum selhe ; ba, basilar artery; ch, chorda ; schb, cartilaginous base of the skull; cm. epithelium of oral cavity.

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At an early period an evagination from the between-braiii (figs. 247, 249), called the infundibulum (tr), has grown out toward the sac of the hypophysis and applied itself to the posterior wall of the latter, which it has folded in toward the anterior or opposite wall.

This first stage is followed by a second, in which the sac and the adjoining end of the infundibulum are metamorphosed into the two lobes of the complete organ already mentioned.

The sac begins (in Man in the second half of the second month, His) to send out from its surface into the surrounding very vascular connective tissue hollow tubules (the tubules of the hypophysis] (figs. 248, 249 hy\ These are then detached from the walls of the sac, by becoming enclosed on all sides by vascular connective tissue. In this respect the process of development agrees in the main with that of the thyroid gland, only that the spherical follicles are here represented by tubular structures. After the entire sac has been resolved into a large number of small, tortuous tubules provided with narrow lumiiia, the lobe thus produced applies itself closely to the lower end of the infundibulum, with which it becomes united by means of connective tissue.

The end of the infundibulnni itself is transformed in the lower Vertebrates into a small lobe of the brain, in which, moreover, ganglionic cells and nerve-fibres can be identified. In the higher Vertebrates, on the contrary, no trace of such histological elements can be detected in the posterior lobe of the hypophysis, which in these- forms consists of closely packed spindle-cells, and thus acquires a close resemblance to a spindle-cell sarcoma.

Development of the First or Fore-Brain Vesicle

The most important changes, the comprehension of which is in part attended with serious difficulties, take place in the vesicle of the fore-brain or cerebrum. It is divided (fig. 250), even at the time of its formation, as has already been mentioned, into a right and a left portion, owing to the fact that its wall becomes infolded from in front and from above by means of a vertical process of the connectivetissue envelope of the brain, the primitive falx. The two portions, the vesicles of the hemispheres (kms), come close together, being separated by only the narrow longitudinal or interpallial fissure (msp), which is filled up by the falx, so that their median surfaces become mutually flattened, whereas their lateral and under surfaces are convex. Where the plane and convex .surfaces are continuous with each other there is a sharp bend in the mantle (Mantelkante).

The vesicles of the hemispheres at first have thin walls formed of several layers of spindleshaped cells (fig. 251, i) and each encloses a large cavity, the lateral ventricle (fig. 251), which is derived from the central canal of the neural tube. Inasmuch as these have been reckoned by the earlier authors as the first and second ventricles, it is plain why the cavities of the betweeii-brain and medulla oblongata are respectively designated as the third and fourth ventricles. In Man, during the earlier months, each lateral ventricle is in communication with the third ventricle by means of a wide opening, the primitive foramen of MONRO (figs. 239 ML and 254 m).

Anterior to the foramen of MONRO lies the part of the wall of the cerebrum which was infolded by the development of the great interpallial fissure : on the one hand it effects the anterior union of the walls of the two hemispheres ; on the other it bounds the third
ventricle in front, and is therefore called the anterior closing plate (lamina
terminalis). It is continuous
below with the anterior wall
of the infundibulum of the
between-brain.

In the further development of each vesicle of the hemispheres four processes are intimately associated : ( 1 ) an extraordinary growth and an enlargement in all directions resulting from it ; (2) an infolding of the wall of the vesicle, so that externally there arise deep clefts (the fissures), and

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Fig. 251. Brain of a human embryo of three months, after KOLLIKER. Natural size.

internally projections into the lateral ventricles; (3) the development of a system of commissures, by means of which the right and left hemispheres are brought into closer union (corpus callosum and fornix) ; (4) the formation of furrows that cut into the cortex of the cerebrum more or less deeply from the outside, but cause no corresponding internal projections in the wall of the ventricle.

As regards its general features, the embryonic growth of the cerebral vesicles is especially characterised by an enlargement backward. In the third month the posterior lobe already completely overlies the optic thalamus (fig. 242) ; in the fifth month it begins to extend over the corpora quaclrigemma (fig. 241), which it entirely covers up in the sixth month. From there it spreads over the cerebellum (fig. 256). The cerebrum is not characterised in all Mammals by such an extraordinary growth as in Man ; comparative anatomy teaches rather that the stages of development of the human brain in different months here described, are met with in other Mammals as permanent conditions.

In some animals the posterior margins of the hemispheres extend as far as the corpora quadrigemina ; in others they cover these more or less completely ; in others, finally, they have grown over the cerebellum more or less. On the whole, the increase in the volume of the cerebrum, which is so varied in Mammals, goes hand in hand with an increase in intelligence.

The vesicles of the hemispheres undergo additional complication (in Man in the course of the second and third months), owing to infoldings of their thin walls, which still enclose a large cavity. As a result of this there arise on the outer surface deep furrows, which separate large areas from one another and which have been designated as total furrows or fissures by His, who has rightly estimated their importance in the architecture of the brain. Corresponding to the f urrows which are visible on the outer surface, there are more or less prominent elevations on the inner surface of the lateral ventricles, by means of which the latter become narrowed and reduced in size. The total furrows of the cerebral hemispheres are the fissure of SYLVIUS (fossa Sylvii), the arcuate fissure, embracing the hippocampal fissure (fissura hippocampi), the fissura choroidea, the fissura calcarina, and the fissura parieto-occipitalis. The elevations produced by them are called the corpus striatum, fornix and pes hippocampi, tela choroidea and calcar avis. A prominence which in the embryo corresponds to the fissura parieto-occipitalis, becomes obliterated in the adult by a considerable thickening of the wall of the brain, so that no permanent structure results from it.

The fissure of SYLVIUS (fig. 252 Sy.g) is the first one formed. It appears as a shallow depression of the convex outer surface at about the middle of the lower margin of each hemisphere. The part of the wall which is thus depressed becomes considerably thickened (figs. 243, 251 cst, and 254 st), and forms an elevation on the floor of the cerebrum projecting into its cavity, the corpus striatum, in which several nuclei of gray matter are developed (the nucleus caudatus, the nucleus lentiformis, and the claustrum). Inasmuch as the elevation lies at the base of the brain and forms the direct forward and lateral continuation of the optic thalamus, it is regarded as belonging to the brain-stalk, and is distinguished as the stalk part of the cerebral hemispheres in distinction from the remaining portion or mantle part. The outer surface of the stalk part can be seen from the outside for a time, as long as the Sylvian fissure is still shallow (fig. 252

Fig. 252. Lateral view of the brain of a human embryo during the first half of the fifth month, after MIHALKOVIC.S. Natural size.

but it then becomes entirely overgrown and hidden by the edges of the gradually deepening fissure. Later this surface acquires in the embryo several cortical furrows and becomes the island of HELL (insula Reilii), or the central lobe (Stammlappen).

The mantle portion, as it enlarges, spreads out uniformly around the island of REIL, as though about a fixed point, and surrounds it in the form of a half-ring open below (fig. 252) ; on this account it has received the name ring-lobe. Even now the regions of the four chief lobes into which the convex surface of each hemisphere is subsequently divided can readily be distinguished, although they are not yet sharply limited. The end of the half -ring which is directed forward and lies above the fissure of SYLVIUS (tiy.y) is the frontal lobe (stl) ; the opposite end, which embraces the fissure behind and below, is the temporal lobe (schl.l) ; the region lying above and connecting the two is the parietal lobe (schei.l). A prominence which is developed from the ring-lobe backward becomes the occipital lobe (hi).

The lateral ventricle has also become altered and corresponds to the external form of each hemisphere (fig. 253). It also assumes the shape of a half -ring, which lies above and surrounds the corpus striatuin (cst) that part of the wall of the vesicle which is forced inward by the fissure of SYLVIUS. Subsequently, when the individual lobes of the hemispheres are more sharply differentiated from one another, the lateral ventricle also undergoes a subdivision corresponding to the lobes. It becomes slightly enlarged at both ends, in front into the anterior cornu occupying the frontal lobe, behind and below into the inferior cornu of the temporal lobe. Finally, from the half ring there is developed a small evagination, the posterior cornu, which extends backward into the occipital lobe. The region lying between the horns is narrowed and becomes the cella media.

All the fissures hitherto mentioned, except that of SYLVIUS, are developed on the plane [median] surface of the vesicle of the hemisphere.

At a very early stage in Man in the fifth week (His) there arise on this wall of the hemisphere two furrows running almost parallel with the edge or bend of the mantle, the arcuate or hippocampal fissure and the fissure of the choroid plexus (Jissura hippocampi andjfoswra choroidea) ; both conform very closely in their direction to the ringlobe, and, like it, with crescentic form embrace from above the stalk part of the cerebrum, the corpus striatum. They begin at the foramen of MONRO and extend from there to the tip of the temporal lobe, forming the boundaries of a region known as the ma/rginal arch (Randbogen) ; this projects as a thickening on the median surface of the hemisphere, and takes part in the development of the connnissural system. The imaginations of the median wall of the ventricle, caused by the fissures, the hippocampal fold and the fold of the lateral choroid plexus, are best understood by removing in an embryo the lateral wall of the hemisphere, so that one can survey the inner surface of the median wall of the still very spacious and ring -like lateral ventricle (fig. 253). The cavity is then seen to be partly filled with a reddish frilled fold (agf), which lies in the form of a crescent on the upper surface of the corpus striatum (cst). In the region of the fold the wall of the brain undergoes changes similar to those in the roof of the medulla oblongata and of the vesicle of the between-brain

(figs. 254 2*1 and 255 agf}. Instead of thickening and developing nerve-substance, it becomes attenuated, and is transformed into a single layer of flat epithelial cells, which are firmly united with the pia mater. The latter then becomes very vascular along
Fig. 253. Lateral view of the brain of an embryo Calf 11 , r i i i

the entire fold, and grows into the lateral ventricle in the form of tufts, which carry the epithelium before them. In this way the lateral choroid plexus arises (fig. 254 pi], which afterwards, in the adult, fills a part of the cella media and inferior cornu. It begins at the foramen of M o N R o (fi g.

cst

5 cm, long. The lateral wall of the hemisphere has been removed. After MIHALKOVICS. Magnified 3 diameters.

it is continuous with the anterior unpaired choroid plexus which has arisen in the roof of the between-brain vesicle. If the delicate vascular pia mater is drawn out from the choroid fissure, the wall of the brain, which is reduced to a thin epithelium, is at the median wall of same time destroyed, and there is produced in the the hemisphere a gaping fissure, which extends from the foramen of MONRO to the tip of the temporal lobe and leads from the outside into the lateral ventricle. This is the lateral cerebral fissure, or the great fissure of the hemispheres (fissura cerebri transversa). In a preparation made in the manner described the hippocampal fold is to be seen at a short distance from the choroid plexus and parallel to it (figs. 253 and 255 aw/" and fig. 254 k). This increases in size toward the apex of the inferior cornu, and in the completely formed brain produces the cornu Ammonis or pes hippocampi. Consequently that part of the lateral ventricle enclosed in the temporal lobe becomes (as the result of two infoldings of its median wall) restricted by two jections, choroid plexus and the cornu Ammonis. As in the betweenbrain and medulla o b 1 o ngata, the epithelial covering of the choroid plexus is continuous with the thicker nerve-su Instance of the Ammonis. The
transition is effected by means of a thin medullary plate, which in anatomy is described as the fimbria.

Fig. 254. Transverse section through the brain of an embryo Sheep 2 -7 cm. in length, after KOLLIKER.

The section passes through the region of the foramen of MONRO. st, Corpus striatum; m, foramen of MONRO; t, third ventricle; pi, plexus choroideus of the lateral ventricle ;/, falx cevebri ; th, deepest anterior part of the optic thalamns ; ch, chiasma ; o, optic nerve ; c, fibres of the crus cerebri ; /<, hippocampal fold ; p, pharynx; sa, presphenoid ; , orbito-sphenoid ; s, part of the roof of the brain at the junction of the roof of the third ventricle with the lamina terminalis ; I, lateral ventricle.

Inasmuch as the occipital lobe with its cavity develops as an evagination of the ring-lobe, the fissura calcarina belonging to it is therefore developed somewhat later than the arcuate fissure (fig. 241 fc). It appears at the end of the third month as a fissure branching off from the latter, and runs in a horizontal direction until near the apex of the occipital lobe. It invaginates the median wall of the lobe and produces the calcar avis, which invades the posterior cornu in the same way as the hippocampus major (cornu Ammonis) does the inferior cornu. At the beginning of the fourth month the fissura occipitalis (fig. 241 fo] is added to it. The latter rises from the anterior end of the fissura calcarina in a vortical direction to the bent rim of the mantle (Mantelkante), and sharply separates the occipital and parietal lobes from each other.

A third factor of great importance in the development of the cerebrum is the formation of a system of commissures, which supplements the connection of the two cerebral vesicles, at first effected by the embryonic lamina terminalis only. Those investigators who have occupied themselves with these difficult matters assert that in the third embryonic month fusions take place between the facing median walls of the hemispheres. These fusions begin in front of the foramen of MONRO within a triangular area. The fusions in this region occur only at the periphery, not in the middle of the area. Three parts of the brain of the adult are thus produced : in front, the genu of the corpus callosum, behind, the columns of the fornix, and between them, the septum pellucidum ; the latter contains a fissurelike cavity, in the region of which the contiguous walls of the hemispheres, here very much attenuated, have remained separated from each other. Consequently the cavity just mentioned the ventriculus septipettucidi [or fifth ventricle] ought not to be placed in the same category with the other cavities of the brain ; for while the latter are derived from the central canal of the embryonic neural tube, the former is a new production, which has arisen by the enclosure of a portion of the space I} 7 ing outside the brain between the two hemispheres the narrow interpallial fissure.

A further enlargement of the commissural system is accomplished in the fifth and sixth months. The fusion now proceeds still farther, advancing from in front backwards, and involves that region of the median walls of the hemispheres which, situated between the arcuate fissure [above] and the fissure of the choroid plexus [below], has already been described as the marginal arch (Randbogen). By fusion of the anterior part of the marginal arch with its fellow of the opposite side, which process takes place as far as the posterior limit of the between-brain, there arise the body of the corpus callosum and the splenium, as well as the underlying fornix. The furrow bounding the corpus callosum above (sulcus corporis callosi) is therefore the anterior part of the arcuate furrow, whereas the posterior portion, that of the temporal lobe, is subsequently known as the fissura hippocampi.

The structure of the cerebrum is completed by the appearance of numerous cortical furrows. These differ in rank from the total furrows already described, because they are confined to the outer surface of the brain and do not cause corresponding projections into the ventricles. Their formation begins as soon as the wall of the brain becomes greatly increased in thickness by the development of white medullary substance, which occurs during and after the fifth month. This is due to the fact that the gray cortex with its ganglionic cells increases more rapidly in superficial extent than the white substance and is therefore raised into folds, the cerebral convolutions or gyri, into which only thin processes of white substance penetrate. At first, therefore, the furrows are quite shallow; they become deeper in proportion as the hemispheres become thicker and the cortical folds project farther outward.

Of the numerous furrows which the completely formed brain presents, some appear during the development earlier, others later. Thus they acquire different values in the architecture of the cerebral surface. For "the earlier a furrow appears the deeper it liecomes, the later it appears the shallower it is " (PANSCH). The first are therefore the more important and constant ones, and are fittingly to be distinguished as chief or primary fwrrow8 from the subsequently formed and more variable secondary and tertiary furrows. They begin to appear at the commencement of the sixth month. The first of them to appear is the central furrow (fig. 256 c/"), which is one of the most important, since it separates the frontal and parietal lobes from each other. " In the ninth month all of the chief sulci and convolutions are formed, and since at this time the secondary sulci are still wanting, the brain during the ninth month presents a typical illustration of the sulci and convolutions " (MiHALKOVlCs).

Very great differences exist between the different divisions of Mammals in the extent to which the sulci of the cerebrum are developed. On the one hand are the Monotremes, Insectivores, and many Rodents, whose cerebrum also usually less developed in other features possesses \\, smooth surface, and thus, as it were, remains permanently in the fietal condition of the human brain. On the other hand the brains of the Carnivores and I'rimates, owing to I he great number of their convolutions, approach more closely to the human brain.

Fig. 256. Brain of a human embryo at the beginning of the eighth month, after MIHALKOVICS. Threefourths natural size.

Finally, in treating of the development of the cerebrum there is still to be considered an appendage to it, the olfactory nerve. This part, as well as the optic nerve, is distinguished from the peripheral
nerves by its entire development, and must be considered as a specially modified portion of the cerebral vesicle. The older designation of nerve is therefore now more frequently replaced by the more appropriate name of olfactory lobe (lobus olfactorius, rhinencephalon). Even at an early stage -in the Chick on the seventh day of incubation, in Man during the fifth week (His) -there is formed on the floor of each frontal lobe at its anterior end a small evagination, which is directed forward (figs. 240, 241 rn). This gradually assumes the form of a club, the enlarged end of which, the part lying 011 the cribriform plate of the ethmoid bone, is designated as the bulbus olfactorius. The bulbus encloses a cavity which is in communication with the lateral ventricle.

During the first month of development the olfactory lobe, even in Man, is relatively large and provided with a central cavity. Later it begins to diminish somewhat, the sense of smell being only slightly developed in Man ; its growth is arrested and at the same time its cavity also disappears. In most Mammals, on the contrary, whose sense of smell, as is well known, is more acute than that of Man, the olfactory lobe attains a greater size in the adult animal and exhibits more clearly the character of a part of the brain, for it permanently encloses in its bulb a cavity, which frequently (Horse) is even in connection with the anterior cornu by means of a narrow canal in the tractus olfactorius.

The olfactory lobe (Lol + Tro) attains an extraordinary development (fig. 257) in the Selachia, in which it exceeds in size the between-brain. (ZH] and mid-brain (MH . In the Selachians two long hollow processes (tractus olfactorius, Tro) extend out from the anterior end of the little-developed cerebrum and terminate at a considerable distance from the fore-brain in two large hollow lobes, that are sometimes provided with furrows (Lol).

The Development of the Peripheral Nervous System

Although it is easy to follow the development of the brain andspinal cord, the investigation of the origin of the peripheral nervous system is very dimcult, for it requires the study of histological processes of the most subtle nature the first appearance of non-medullated nerve-fibres and the method of their termination in embryos composed of more or less undifferentiated cells. One who knows how dimcult it is even in the adult animal to follow non-medullated iierve-fibrillse in epithelial layers or in non-striate muscle-tissue, and to get a clear idea of their method of termination, will understand that many, and indeed the most interesting, questions in regard to the development of the peripheral nerves are not yet ripe for discussion, because the observations necessary for their settlement are still wanting. There is only one point which is entirely clear. That concerns the development of the spinal ganglia, which His and BALFOUR independently of each other were the first to recognise, the one in the Chick, the other in Selachians. Since then numerous investigations embracing different groups of Vertebrates have been published on this subject by HENSEN, MILNES MARSHALL, KOLLIKER, SAGEMEHL, VAN WIJHE, BEDOT, ONODI, BERANECK, EABL, BEARD, KASTSCHENKO, and others.

The Development of the Spinal Ganglia

The development of the spinal ganglia in the spinal cord is very easily followed. It begins just at the time the medullary groove closes to form a tube (fig. 258 A and B). At this time a thin ridge of cells (spy 1 , spy] one or two layers deep grows out of the neural tube on either side of the line of fusion, and, passing outward and downward, inserts itself between the tube and the closely investing primitive epidermis. In this way it reaches the dorsal angle of the primitive somites (us), which are by this time well
developed. Then the neural crest, as BALFOUR names it, or the ganglionic ridge, as SAGEMEHL calls it, is divided up into successive regions. For the tracts which alternate with the primitive segments lag behind in their growth, while the parts lying opposite the middle of segments grow more vigorously, become thickened, and at the same time advance farther ventrad, penetrating between primitive segment and neural tube.

Frontal sections furnish very instructive views of this stage. Fig. 259 exhibits such a section, taken from SAGEMEHL'S work. Inasmuch as the longitudinal axis of the Lizard embryo employed for the sections was greatly curved, the five segments seen in the section are cut at different heights, the middle one deeper than the two preceding and the two following. In the middle segment the fundament of the ganglion (spA 1 ) is differentiated and it is bounded by blood-vessels in front and behind, whereas in the segments that are cut more dorsally, near the origin of the ganglia from the neural tube, the fundaments are still connected with one another. This connection appears to be most conspicuously developed and most persistent in the case of the Selachians ; it has been called the longitudinal commissure by BALFOUR. Outside the ganglia are found the primitive segments (mp, nip'}, each of which at this time still exhibits within it a narrow fissure.

Fig. 258. A, Cross section through an embryo of Pristiurus, after EABL. The primitive segments are still connected with the remaining portion of the middle germ-layer. At the region of transition there is to be seen an outfolding, sk, from which the skeletogenous tissue is developed, ch, Chorda ; spg, spinal ganglion ; mp, muscle-plate of the primitive segment ; nch, subchordal rod ; ao, aorta ; ik, inner germ-layer ; 2nb, parietal, rmb, visceral middle layer.

B, Cross section through a Lizard embryo, after SAGEMEHL. rm, Spinal cord ; spy, lower thickened part of the neural ridge ; spy', its upper attenuated part, which is continuous with the roof of the neural tube ; us, primitive segment.

In a monographic treatment of the peripheral nervous system BEARD differs from the preceding account, in which BALFOUR, KOLLIKER, EABL, HENSEN, SAGEMEHL, KASTSCHENKO, and others agree. He believes that the fundaments of the ganglia arise, not out of the neural tube, but out of the deeper cell-layers of the adjacent part of the outer germ-layer. He finds that they are from the beginning separated from each other and segmentally arranged. According to him, moreover, they make their appearance earlier than is stated in the preceding account ; for they are already recognisable as especially thickened places in the outer germ-layer at the tight and left of .the neural plate when the latter first begins to be bent inward.

Subsequently, upon the closure of the neural tube, the ganglionic cells corne to lie between the raphe and the primitive epidermis. From here they grow down ventrally at the sides of the brain and spinal cord.

BEARD approximates in his results the conception first expressed and subsequently maintained by His. For His derives the ganglionic ridge, not from the raphe of the neural tube, but from a neighboring part of the outer germ-layer, which he names intermediate cord (Zwischenstrang). The accuracy of BEARD'S description is, however, positively denied by KABL and KASTSCHENKO.

Different views are entertained concerning the further changes which take place in the fundaments of the spinal ganglia :
According to His and SAGEMEHL the separate ganglionic fundaments are completely detached from the neural tube, and for a time lie at the side of it without any connection with it whatever. Secondarily a union is again established, through the development of the dorsal nerve-roots, by the formation of nerve-fibrillre, which either grow out from the spinal cord into the ganglion, or from the ganglion into the spinal cord, or in both directions. SAGEMEHL favors the first view, His the last. All other investigators maintain that the fundaments of the ganglia, while they increase in size and become spindle-shaped, are permanently united with the neural tube by means of slender cords of cells which are metamorphosed into the dorsal roots. If the latter view is correct, the dorsal roots of the nerves must in time alter their place of attachment to the neural tube by moving from the raphe laterally and ventrally. The discrepancy of these accounts is connected with the different interpretations which exist concerning the development of the peripheral nerves in general.

Fig. 259. Frontal section of a Lizard embryo, after SAGEMEHL.

spk;, Spinal cord; neural ridge with thickenings that are converted into the spinal ganglia ; mp', the part of the primitive segment that produces the muscle-plate ; mp, outer layer of the primitive segment.

The Development of the Peripheral Nerves

When one reviews the various opinions which have been expressed concerning the development of the peripheral nerves, it is found that there are in the literature two chief opposing views. The greater number of investigators assume that the peripheral nervous system is developed out of the central, that the nerves grow forth from the brain and spinal cord uninterruptedly until they reach the periphery, where for the first time they effect a union with their specific terminal organs. The outgrowth of the nerves from the spinal cord was first asserted for the ventral roots and conjectured for the dorsal ones by BIDDER UNO KUPFFER. Their conclusions have since been adopted by KOLLIKER, His, BALFOUR, MARSHALL, SAGEMEHL, and others. However, views concerning the method of the formation of the nerve-fibres are not in agreement.

According to KUPFFER, His, KG'LLIKER, SAGEMEHL, and others the outgrowing nerve-Jibres are processes of ganglionic cells located in the central organ, which must grow out to an enormous length in order to reach their terminal apparatus. There are at first no cells or nuclei among them. These are furnished secondarily by the surrounding connective tissue. According to the accounts of KOLLIKER and His, cellular elements from the mesenchyme approach the bundles of nerve-fibrillse, surround them, and then penetrate into the interior of the nervous stem, at first sparingly, afterwards more abundantly, and form around the axis-cylinders the sheaths of SCHWANN.

On the other hand, BALFOUR defends most positively the doctrine that cells which migrate out of the spinal cord along with the nerves share in the development. In his " Treatise on Comparative Embryology " [vol. ii., p. 372] he remarks upon this subject : " The cellular structure of embryonic nerves is a point on which I should have anticipated that a difference of opinion was impossible, had it not been for the fact that His and KOLLIKER, following REMAK and other older embryologists, absolutely deny the fact. I feel quite sure that no one studying the development of the nerves in Elasmobranchii with well-preserved specimens could for a moment be doubtful on this point." Of the more recent investigators VAX WIJHE, DOHRN, and BEARD side with BALFOUR.

HENSEN has taken an entirely different view on the question of the origin of the peripheral nervous system, one which differs from that of KUPFFER, HTS, and KOLLIKER, as well as from that of BALFOUR. He opposes the doctrine of the outgrowth of nerve-fibres chiefly from physiological considerations. He can think of no motive which is capable of conducting the nerves that gro\v out from the spinal cord to their proper terminations which shall cause, for example, the ventral roots always to go to muscles, the dorsal roots to organs that are not muscular, and shall prevent confusion taking place between the nerves of the iris and those of the eye-muscles, between the branches of the trigeminus and the acusticus or facialis, etc. Therefore HENSEN maintains on theoretical grounds that it is necessary to assume that " the nerves never grow out to their terminations, but are always in connection with them" According to his view, which he endeavors to support by observations, the embryonic cells are for the most part united with one another by means of fine connecting filaments. He maintains that when a cell divides the connecting thread also splits, and in this manner there arises " an endless network of fibres." Out of these the nervetracts are developed, while other parts of the network degenerate.

The reasons given by HENSEN are certainly worthy of great attention. With further reflection on the subject they are easily added to. If the nerves grow out to their terminal apparatus, why do they not take the most direct course to their destination, why are they often compelled to pursue circuitous paths, and why do they enter into the formation of complicated plexuses of the greatest variety ? whence are the ganglionic cells that are found to be developed in considerable numbers even in the peripheral nervous system in different organs, especially in the sympathetic nerve ? In order to make progress in this difficult field the peripheral nervous system of Invertebrates mast be taken into account more than it is at present, and in the investigation of embryos not only series of sections but also other histological methods (surface-preparations of suitable objects together with staining of the nerve-fibrillce, isolation of the elements preceded by maceration and staining) must be employed.

Having thus sketched out the various standpoints taken by numerous investigators on the question of the source of the peripheral nervous system, I give a number of observations that have been made upon the development of certain nerves. These relate to the development of :

The ventral and dorsal roots of the nerves ;

Certain large peripheral nerve-trunks, as the nervus lateralis ;

The nerves of the head and their relation to the spinal nerves.

(1) Of the roots of the nerves the anterior [ventral] are demonstrable earlier. There may be distinguished three stages in their development.

The first stage has been observed by DOHRN and VAN WIJHE in Selachian embryos. At a time when the neural tube has not yet developed any mantle of nervous substance, and the muscle-segment still lies very close to it, there arises between the two a connection in the form of a very short protoplasmic cord. The fundament of the nerve is therefore, as VAN WIJHE remarks, ab origine near its muscle-complex, from which it never separates. Soon after this it is elongated by the removal of the muscle-segment farther from the neural tube ; it increases in thickness and now encloses numerous nuclei, and possesses therefore a cellular composition, a condition which I shall designate as second stage.

There is a difference of opinion as to the cells which make their appearance in the fundament of the nerve. Whereas TVOLLTKER His, and SAGEMEHL recognise in them immigrated connective-tissue elements, which are destined to form simply the envelopes of the nerves, BALFOUR, MARSHALL, VAN WIJHE, DOHRN, and BEARD maintain that they migrate out from the spinal cord and share in the development of the nerves themselves. BEARD even derives the motor terminal plate from them. Soon after, as is asserted, connective-tissue cells from the surrounding mesenchyme become associated with the nerve-cells derived from the spinal cord and ordinarily become indistinguishable from them.

Finally, in the third stage the cellular fundament of the motor root acquires a fibrillar condition (fig. 260 vw\ and it now becomes possible to trace the origin of the nerve-fibrillse in the spinal cord from groups of embryonal ganglionic cells or neuroblasts (His).

The formation of the nerve-fibrillae is also a subject of controversy, as has already been stated and as will be farther explained in this connection. According to the view of most observers, the nervefibrillae the future axis -cylinders are formed as processes of gang1 ionic cells of the spinal cord, the free ends of which grow out from the surface of the latter until they reach their terminal organs (KUPFFER UND BlDDER, KoLLIKER, HlS, SAGEMEHL). Sticll aCCOUllts

are given especially for the development of the motor roots in the higher Vertebrates.

According to the opinion of DOHRN and VAN WIJHE, on the contrary, the nerve-fibrillse arise in situ, as products of differentiation, from the protoplasm of the cords of cells by means of which musclesegnierit and spinal cord are already united. They do not need to seek out the terminal organ, since there exists already a protoplasmic union with it. They arise in a manner similar to that in which the nmscle-fibriJlse do from the plasma of their, muscle-cells^

I desire to lay pavticnlar .shvss upon the observations oC DOTIRN and VAN WiJHK, because they harmonise with the theoretical views which I have formed as the result of investigations on Invertebrates. As 1 have in several articles endeavored to establish, protoplasmic connections of the cells are the foundation out of which the nerve-fibrillse are developed. The formation of a specific nervous system is preceded by a protoplasmic union of cells, which is effected at a time when the central and terminal nervous organs are still in the immediate vicinity of each other.

The dorsal roots become visible somewhat later than the ventral roots ; there are formed fibrillce which unite the upper [dorsal] end of the spinal ganglion with the side of the spinal cord.

(2) GOTTE, SEMPER, WIJHE, HOFFMANN, and BEARD have made concerning certain nerves the noteworthy statement which has been called in question by some observers (BALFOUR, SAGEMEHL) that the epidermis participates in their formation. In Amphibian larvae and Selachian embryos the posterior end of the nervus lateralis vagi in process of development is completely fused with the primitive epidermis, which is thickened in the lateral line (fig. 262 nl). Somewhat farther forward the nerve is detached but still lies in close contact with the primitive epidermis, whereas in the vicinity of the head it is situated deeper and lies between the muscles. At the places where the nerve has become separated from the primitive epidermis, it remains in connection with the fundaments of the lateral organs by means of fine accessory branches only. Similar observations have also been made in the case of many of the branches of other cranial nerves in Selachian embryos. WIJHE sees, for example, a short branch of the n. facialis near its emergence from the brain so fused with a thickened portion of the epidermis composed of cylindrical cells, that it is impossible to say whether at the place of transition the cell-nuclei belong to the nerve or to its terminal organ. During a more advanced stage the older part of the nerve is detached from the terminal organ, sinks into the depths, becoming separated from the skin by ingrowing connective tissue, and remains united with the terminal organ only through fine accessory branches. The persistently growing younger end of the nerve still continues to be connected with the epidermis.

Also in the case of the higher Vertebrates similar conditions have been observed by BEARD, FRORIEP, and KASTSCHENKO. They find the ganglionic fundaments of the facialis, glossopharyngeus, and vagus at the dorsal margin of the corresponding visceral clefts for a long time broadly fused with the epithelium, which is thickened and has become depressed into a pit. In these connections they discern the fundaments of branchial sensory organs, which no longer attain to complete development. Also FRORIEP, on the strength of his own observations, holds as admissible the interpretation that at those places where fusion occurs formative material passes out of the epidermis into deeper parts to share in the formation of nervous tracts. BEARD expresses himself still more precisely to the effect that the sensory nervous elements of the whole peripheral nervous system arise as differentiations from the outer germ-layer, independently of the central nervous system.

The accounts here given concerning a connection, in early stages of development, of certain nerve-trunks with the outer germ-layer, appear to me to afford an indication in favor of the hypothesis expressed by my brother and me, that the sensory nerves of the Vertebrates may have originally been formed out of a sub-epithelial nervous plexus, such as still exists in the epidermis of many Invertebrates.

(3) The investigations of the last few years, which have been carried out especially by BALFOUR, MARSHALL, KOLLIKER, WIJHE, FRORIEP, RABL, and KASTSCHENKO, have produced important results concerning the development of the cranial nerves, their relations to the head-segments and their value as compared with spinal nerves. On the brain, as well as on the spinal cord, there arise roots, some of which are dorsal, some ventral. Even at the time when the brain-plate is not yet fully closed into a tube (fig. 261), there is formed on either side, at the place of its bending over into the primitive epidermis, a neural ridge (vg\ which begins rather far forward and may be traced on serial sections uninterruptedly in a posterior direction, where it is continuous with the neural ridge of the spinal cord. When, somewhat later, the closure and the detachment of the brain -vesicles from the primitive epidermis has taken place, the ridge lies on the roof of the vesicles and is fused with them in the median plane. The most of the cranial nerves namely, the trigerninus with the Gasserian ganglion, the acusticus and facialis with the ganglion acusticum. and probably also the ganglion geniculi, and the glossopharyngeus and vagus with the related ganglion jugulare and g. nodosum are differentiated out of this fundament in the same manner as the dorsal roots of the spinal nerves. The nerves, which emerge dorsally, afterwards shift their origin downward along the lateral walls of the brain -vesicles toward the base of the latter.

All the remaining uneiiumerated cranial nerves oculomotorius, trochlearis, abducens, hypoglossus, and accessorius are developed independently of the neural ridge, as individual outgrowths of the brain- vesicles nearer their base, and are comparable with the ventral roots from the spinal cord.

FROEIEP finds that the hypoglossns in Mammals possesses dorsal roots, with small gang-Home fundaments, in addition to ventral roots. The latter subsequently undergo degeneration.

The agreement between cranial and spinal nerves which is expressed in this method of development, becomes still greater and acquires a further significance from the fact that in the head also the nerves can be assigned to separate segments in much the same manner as in the trunk. In this particular the conditions are clearest in the Selachians, where, in fact, the head-segments have been most thoroughly investigated, so that I limit myself to a statement of the results acquired in this field by WIJHE.

Fig. 261. Cross section through the hind part of the head of a Chick embryo of 30 hours, after BALFOUR.

According to WIJHE nine * segments are distinguishable in the head of Selachians. To the first segment belongs the ramus

[Recent investigations indicate that the head-segments in Selachians are much more numerous. TRANSLATOR.]

ophthalmicus of the trigeminus and, as motor root, the oculomotorius. The second segment is supplied by the remaining part of the trigeminus and the trochlearis, the latter having a ventral origin. The dorsal roots of the third (and fourth'?) segments are represented by the acustico-facialis, the ventral roots by the abduceiis. The fifth segment possesses only the exclusively sensory gloFsopharyngeus, which arises from the neural ridge. The segments from the sixth to the ninth inclusive are innervated by the vagus and the hypoglossus, the former of which represents a series of dorsal roots, the latter a series of ventral ones.

According to WIJHE'S account, notwithstanding the general agreement, there still exists a considerable difference between the innervation of the cephalic segments and that of the trunk-segments. For in the head the ventral, motor roots (oculomotorius, trochlearis, abducens, hypoglossus) supply only a part of the musculature the eyemuscles and certain muscles that run from the skull to the pectoral girdle ; that is to say, muscles which, as has already been stated, are developed out of the cephalic segments. Other groups of muscles, which arise from the lateral plates of the head, are innervated by the trigeminus and facialis, which have a dorsal origin. Thus the dorsal roots of the nerves in the head would be distinguished from those in the trunk by the important fact that they contain motor as well as sensory fibres. BELL'S law would consequently possess a very limited application for the head-region of Vertebrates, and would have to be replaced by the following law, formulated by WIJHE :
" The dorsal roots of the head-nerves are not exclusively sensory, but also innervate the muscles that arise from the lateral plates, not, however, those from the primitive segments (somites)."
" The ventral roots are motor, but innervate only the muscles of the primitive segments (somites), not those of the lateral plates."
In view of this fundamental difference, I desire to express a doubt whether there are not after all enclosed in the facialis and trigeminus parts which are established as ventral roots, but have hitherto been overlooked, as in the beginning all the ventral roots in the brain (see BALFOUR) were overlooked.

According to KABL the nerves of the posterior part of the head only o-lossopharyngeus, vagus, accessorius, and hypoglossus can be compared with the type of spinal nerves ; the nerves of the anterior part of the head, on the contrary, the olfactorius, options, trigeminus, together with those of the eyemuscles and the acustico-facialis, belong in a separate category, just as the four most anterior head-segments do.

As is evident from this brief survey, there still exist many unsolved problems in the difficult subject of the development of the peripheral nervous system. Without permitting myself to enter upon a further discussion of the contradictory opinions entertained on this subject, I close this topic with a comparative-anatomical proposition, which appears to me sufficient to furnish the morphological explanation of BELL'S law, or the separate origin of the sensory and motor nerveroots.

In Arnphioxus and the Gyclostomes the motor and sensory nervefibres are completely separated, not only at their origin from the spinal cord, but also throughout their whole peripheral distribution. The former pass at once from their origin in the spinal cord to the muscle-segments ; the latter ascend to the surface to be distributed to all parts of the skin to supply its sensory cells and sensory organs. The separation of the peripheral nervous system into a sensory and a motor portion, ivhich is rigorously carried out in Amphioxus and the Gyclostomes, is explained by the fact that the territories to which their ends are distributed are spatially distinct in their origin, since the sensory cells arise from the outer germ-layer, the voluntary muscles from a tract of the middle germ-layer. Therefore the sensory nervefibres have been developed from the spinal cord in connection with the outer germ-layer, the motor fibres in relation with the musclesegments.

I regard the sub-epithelial position of the sensory nerve-fibres as the original one, just as we find in many Invertebrates the whole peripheral sensory nervous system developed as a plexus in the deepest portion of the epidermis. The important conditions above described according to which many dermal nerves (nervus lateralis, etc., fig. 262 nl) are fused with the epidermis at the time of their origin, and only subsequently become detached from it and sink deeper into the underlying mesenchyme appear to me to indicate that such a position was the primitive one in the case of Vertebrates also.

I look upon the union of the sensory and motor nerve-fibres into mixed trunks (which occurs soon after their separate origin from the spinal cord, in the case of all Vertebrates except Amphioxus and the Cyclostomes) as a secondary condition, and maintain that it is caused especially by the following embryological influences : by the change in the position of the spinal cord and the muscular masses, and by the great increase in the amount of the connective substances.

Fig. 262. Cross section through the anterior part of the trunk of an embryo of Scyllium, after BALFOUR. Between the dorsal wall of the trunk and its ventral wall, where the attachment of the stalk of the yolk-sac is cut, there is stretched a thick richly cellular mesentery, which completely divides the body-cavity into right and left halves. Within the mesentery the duodenum (du), from which the fundament of the pancreas (pan) is given off dorsally and the fundament of the liver (hp.d) ventrally, is twice cut through. In addition, the place where the vitelline duct [umbilical canal] (HHIC) joins the duodenum is visible.

Since the spinal cord comes to lie in deeper layers of the body far away from its place of origin, the dermal nerves must follow it, and therefore their origins are correspondingly farther separated from their terminations. Since also, on the other hand, the muscleplates grow around the neural tube, certain motor and sensory nerve-cords are brought near to each other in their passage to their peripheral distribution. And this will occur especially in all cases where the motor and sensory peripheral terminations lie at a great distance from the origin of the nerves out of the spinal cord, as, for example, in the case of the limbs. The mutual approximation of sensory and motor nerve-tracts thus brought about will finally lead to the formation of common tracts, according to the same principle of simplified organisation in accordance with which the blood-vessels also adapt themselves closely to the course of the nerves.

The Development of the Sympathetic System

The development of the sympathetic nervous system has as yet been investigated by only a few observers. BALFOUR first announced that it arose in connection with the cranial and spinal nerves, and therefore was, like the latter, really derived from the outer germlayer. In the Selachians he found the sympathetic ganglia (fig. 262 sy.y] as small enlargements of the chief trunks of the spinal nerves (sp.n) a little below their ganglia (s/>.#). In older embryos, according to BALFOUR'S account, they recede from the spinal ganglia, and then at a later period unite with one another, by the development of a longitudinal commissure, into a continuous cord (Grenzstrang).

The origin of the sympathetic system has been the most thoroughly studied by ONODI in researches covering several classes of Vertebrates. According to him the sympathetic ganglia arise directly, as BALFOUR suggested and as BEARD has also lately reiterated, from the spinal ganglia. The ventral ends of the spinal ganglia undergo proliferation, as is best seen in Fishes. The proliferated part detaches itself, and, as fundament of a sympathetic ganglion, moves ventrally. The fundaments of the individual segments are at first separate from one another. The cord (Grenzstrang) is a secondary product, produced by the growing out of the individual ganglia toward each other and the union of the outgrowths. Afterwards the sympathetic ganglia and plexuses of the body-cavity are derived from this part.

Summary

Central Nervous System

The central nervous system is developed out of the thickened region of the outer germ- layer which is designated as the medullary plate.

The medullary plate is folded together to form the medullary tube (medullary ridges, medullary groove).

The lateral walls of the medullary tube become thickened, whereas the dorsal and ventral walls remain thin ; the latter come to occupy the depths of the anterior and posterior longitudinal fissures, and constitute the commissures of the lateral halves of the spinal cord.

The spinal cord at first fills the whole length of the vertebral canal, but it grows more slowly than the latter, and finally terminates at the second lumbar vertebra (explanation of the oblique course of the lumbar and sacral nerves).

The part of the neural tube which forms the brain becomes segmented into the three primary cerebral vesicles (primary forebrain vesicle, mid-brain vesicle, hind-brain vesicle).

The lateral walls of the fore-brain vesicle are evaginated to form the optic vesicles, the anterior wall to form the vesicles of the cerebrum.

The hind-brain vesicle is divided by constriction into the vesicles of the cerebellum and the medulla.

Thus from the three primary brain-vesicles there finally arise five secondary ones arranged in a single series one after the other (a) cerebral vesicle (that of the hemispheres), (6) between-brain vesicle with the laterally attached optic vesicles, (c) mid-brain vesicle, (cl) vesicle of the cerebellum, (e) vesicle of the medulla oblongata.

The originally straight axis uniting the brain-vesicles to one another later becomes at certain places sharply bent, in consequence of which the mutual relations of the vesicles are changed (cephalic flexure, pontal flexure, nuchal flexure). The cephalic or parietal protuberance at the surface of the embryo corresponds to the cephalic flexure, the nuchal protuberance to the nuchal flexure.

The separate parts of the brain are derivable from the live brain-vesicles ; the accompanying table (MiHALKOVlCS, SCHWALBE) gives a survey of the subject.

In the metamorphoses of the vesicles the following processes take place : (a) certain regions of the walls become more or less thickened, whereas other regions undergo a diminution in thickness and do not develop nervous substance (roof-plates of the third and fourth ventricles) ; (b) the walls of the vesicles are infolded ; (c) some of the vesicles (first and fourth) greatly exceed in their growth the remaining ones (bet ween- brain, mid-brain, after-brain, or medulla oblongata).

The four ventricles of the brain and the aqueduct us Sylvii are derived from the cavities of the vesicles.

Of the five vesicles that of the mid-brain is the most conservative and undergoes the least metamorphosis.

The vesicles of the between-brain and after-brain exhibit similar alterations : their upper walls or roof -plates are reduced in thickness to a single layer of epithelial cells, and in conjunction with the growing pia mater produce the choroid plexuses (anterior, lateral, posterior choroid plexus ; anterior, posterior brain-fissure).

The cerebral vesicle is divided by the development of the longitudinal (interpallial) fissure and the falx cerebri into lateral halves, the two vesicles of the cerebral hemispheres.

In Man the cerebral hemispheres finally exceed in volume all the remaining parts of the brain, and grow from above and from the sides as cerebral mantle over the other brain- vesicles (from the second to the fifth inclusive) or the brain-stalk.

In the folding of the walls of the hemispheres there are to be distinguished fissures and sulci.

The fissures (fossa Sylvii, fissura hippocampi, fissura choroidea, fissura calcarina, fissura occipitalis) are complete folds of the wall of the brain, by means of which there are produced deep incisions in the surface and corresponding projections into the lateral ventricles (corpus striatum, cornu Ammonis, fold of the choroid plexus, calcar avis).

The sulci are incisions limited to the cortical portion of the wall of the brain, and are deeper or shallower according to the time of their formation (primary, secondary, tertiary sulci).

In general the fissures appear earlier than the sulci.

The olfactory nerve is not equivalent to a peripheral nervetrunk, but, like the optic vesicle and optic nerve, a special part of the brain produced by an evagination of the frontal lobe of the cerebral hemisphere (lobus or bulbus olfactorius with tractus olfactorius). (Enormous development of the olfactory lobes in lower Vertebrates, Sharks, degeneration in Man.)

Peripheral Nervous System

The spinal ganglia are developed out of a neural ridge (crest), which grows outward and downward from the raphe of the neural tube 011 either side between the tube and the primitive epidermis, and becomes thickened in the middle of each primitive segment into a ganglion.

The spinal ganglia therefore arise, like the neural tube itself, from the outer germ-layer.

The sympathetic ganglia of the longitudinal cord (Grenzstrang) are probably detached parts of the spinal ganglia.

Concerning the development of the peripheral nerve-fibres there are different hypotheses :

First hypothesis. The peripheral nerve-fibres grow out from the central nervous system and only secondarily unite with their peripheral terminal apparatus.

Second hypothesis. The fundaments of the peripheral terminal apparatus (muscles, sensory organs) and the central nervous system are connected from early stages of development by means of filaments which become nervefibres (HENSEN).

Anterior and posterior nerve-roots are developed on the spinal cord separately from each other, one ventrally, the other dorsally.

The cranial nerves arise in part like posterior, in part like anterior roots of spinal nerves.

The following cranial nerves with their ganglia, which are comparable with spinal ganglia, are developed out of a neural ridge which grows out from the raphe of the brain -vesicles : the trigeminus with the ganglion Gasseri, the acusticus and facialis with the ganglion acusticum and g. geniculi, the glossopharyngeus and vagus with the ganglion jugulare and g. nodosum.

The oculomotorius, trochlearis, abducens, hypoglossus, and accessorius are developed like ventral roots of spinal nerves.

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